wohlwill



No. 625,863. Patenfed May'30, I899. E. WDHLWILL.

PROCESS OF OBTAINING CHEMIGALLY PURE GOLD.

(Application filed June 4, 1896.

(No Model.) 2 Sheets-Sheefl l.

TM: nonm's versus c0. PHOTCLLIYHUH WASHINGTON, a. c.

No. 625,863. Patented May 30, 899.

E. WUHLWILL. PROCESS OF OBTAINING GHEMIGALLY PURE GOLD.

(Application filed U'une 4, 1896.) (No Model.)

' 2 Sheets-Sh2et 2.

UNITED STATES PATENT OFFICE,

EMIL WOHLWILL, OF HAMBURG, GERMANY.

PROCESS OF OBTAINING CHE MICALLY- PURE GOLD;

SPECIFICATION forming part of Lears mea No. 625,863, dated May 30, 1899.

Application filed June 4 1896. Serial No. 594,339. (No specimensil To all whom it may concern:

Beit known that I, EMIL WOHLWILL, a subject of the German Emperor, residing at Hamburg, Germany, have invented an Improved Process of Obtaining Chemically-Pure Gold from Fine Gold and Alloys Richin Gold; and I do hereby declare the following to be a full, clear, and exact description of'the'invention.

This invention has relation to the electrolytic extraction of gold from so-called refined gold obtained by affinage or otherwise and which still contains other metals in various proportions or from alloys rich in gold.

The invention has for its object a process whereby the Whole of the gold contained in the material to be used as anode is obtained in the form of chemicallypure gold and whereby the process of extraction can be commercially carried out and other valuable metals alloyed with the gold readily recovered.

As is well known, in the chemical separation of gold from impure gold or from alloys ofgold the gold is dissolved out of the disintegrated impure gold or alloy by means of aqua regia, and from the clear dilute solution the gold is precipitated by means of a suitable precipitant, while in the electrolytic extraction the dissolutionof the gold and its extraction from the solution are effected simultaneously, as is the case in the electrolytic extraction of copper and silver. is necessary during the electrolytic decomposition of the gold solution used as anelectro- I lyte that a quantity of gold equal to that deposited at the cathode be carried into the electrolyte under the influence of the gold ion or compounds thereof separated at the anode,

so that in an uninterrupted extraction the tion of gold by electrolysis that the evolu be avoided.

To this end it of a solution of gold in aqua regia.

tion of gaseous chlorin at the anode should It is furthermore obvious that the electrolytic extraction of the precious metal is most economically effected with currents of comparatively high density, as the dissolution of the anode gold or compound and the precipitation of the pure goldat the cathode are therebymaterially expedited. If, however, a solution of gold chlorid in presence of an anode of gold or of a compound or alloy thereof is electrolyzed under the action of an electric current, and especially of a current of comparatively high density, it will be found that'there is an essential departure in the behavior of the gold from that of copper or silver under like conditions.

While it is practically immaterial in the continuous carrying over of the anode copper or silver to the electrolyte whether the latter is neutral or more or less acid or more'or less concentrated or more or less heated, these factors have a decided influence upon the electrochemical dissolution of gold or its compounds-or alloys, which is more pronounced when currents of comparatively high density are used. Thus, for instance, if a neutral pure gold-chlorid solution of normal temperature is electrolyticallydecomposed in the presence of gold electrodes by the action of a current of as low a density as ten amperes per square meter the free chlorin evolved at the anode will escape without action upon the metal, and this condition is not changed by slightly-acid electrolyte is used, such as may,

for instance, be obtained by the evaporation I have, however, discovered that the disturbing evolotion and escape of chlorin at the anode are not only materially reduced, but absolutely avoided, Whenever the electrolyte contains sufficient hydrochloric acid. I have found that at a given temperature the maximum current density at which the whole of thechlorin evolved at the anode is bound corresponds with the quantity of the hydrochloric acid present in the electrolyte, or vice versa, and thatthis maximum of current density and of chlorin to be bound can be increased in prothe electrolyte is increased and that a further and very material increase takes place when the temperature of the electrolyte is increased relatively to a given percentage of hydrochloric acid therein. Hence if at a given temperature of the electrolyte containing a given quantity of hydrochloric acid the current density is increased beyond the corresponding maximum density an amount of free chlorin corresponding with this increase in current density above the maximum will escape at the anode, which can, however, be prevented either by a corresponding increase in the percentage of hydrochloric acid added to the electrolyte or by increasing the temperature of the latter. My experiments have conclusively proven that in this manner it is possible to continuously operate with comparatively Weak solutions of gold chlorid and with currents of a density as high as one thousand ainperes per square meter and even higher through the combined influence of high temperature and added hydrochloric acid.

The invention therefore consists, essentially, in the electrolytic purification of impure gold and of gold alloys by employing as an electrolyte solutions of chlorid of gold containing hydrochloric acid in sufficient quantity to bind the chlorin evolved at the anode or gold-chlorid solutions of a sufficiently high temperature and containing sufficient hydrochloric acid to bind the free chlorin evolved at the anode.

The fact stated above, that the amount of gold or a compound or alloy thereof dissolved at the anode is proportionate to the amount of hydrochloric acid present in the solution and capable of preventing the escape of gaseous chlorin, will readily be understood from the following: The belief has prevailed that in the known processes of dissolving gold the chlorid (AuOl is obtained; but this cannot be proved in any manner. Aftera thorough examination into the combinations of gold published by Julius Thomsen (Thermochemische (Miersuchungcn, Vol. 3, p. 382, et seq, Leipsic, 1883) there are but two ways of obtaining chlorid of gold, (AuOl First, finelydivided gold is acted upon with gaseous chlorin of from 200 to 300, whereby Au Ol is obtained, which in water splits up into AuCl and AuCl. Second, protochlorid of gold (AuOl) is heated for some time in water, when it splits up in accordance with the following equation:

The AuCl obtained in both cases may be recognized by the fact that it forms with water a brownish-red solution, the thicker layers of which appear black. If this solution is mixed with a suitable quantity of hydrochloric acid, it assumes a yellow color, in that in accordance with the following equation AuOl +HOl:.AuO1 HCl the double chlorid of gold and hydrogen, designated by Berzelius as the acid gold chlorid, is formed. This combination is formed under the evolution of considerable heat, (see Thomsen referred to, Vol. 3, p. 392,) which proves that after the formation of AuOl llOl hydrochloric acid has disappeared, and this is shown by the fact that chlorin is not separated when a concentrated solution of AuGl HCl is reacted upon with nitric acid, (which would occur if free hydrochloric acid were present,) and hence does not dissolve gold. If the solution of Au (El i-I01 is evaporated to dryness, crystals are formed of the composition AuCl HCl+4H O, which give up the 1101 only at higher temperatures, the AuCl being split up into AuOl and 201. Besides this the com pound AuCl llCl has the properties of a mono basic acid, and it is therefore correct to write it HAuCl There can be no doubt that in each direct solution of gold by chemical means this compound HAuCl is simultaneously formed and passes over into the solution. If, for instance, gold is dissolved in aqua regia, a yellow solution is obtained, from which crystals of the given composition AuCl l-I+4H O are obtained. It must therefore be concluded that of each four parts of the reacting hydrochloric acid in the aqua regia three are always used up in the formation of chlorin and therethrough the chlorination of the gold, while the fourth part is used up in an undecomposed condition in the formation of HAuGl so that the reactions which take place may be briefly expressed by the following equations:

1. I'INO l-SHOl:3Gl+2H O+NO. 2. Au+3Cl+I-IOl:HAuOl,.

It may be assumedand this is the general opinion-that the process expressed in Equation 2 comprises two successive reactions, and that the action of the chlorin-evolved from the aqua regia upon the gold will form a neutral gold chlorid in accordance with the following equation:

3. Au+(ll- :AuCl

and that through the excess of undecomposed 4. AuCh-l-HCl HAuCh.

It may, however, be assumed that the formation of the compound I-IAuOl according to Equation 2, is only a single indivisible reaction, so that the neutral chlorid of gold does at no time exist or could be supposed to exist as an intermediate step. Both views do clearly show the reactions that take place in the dissolution of gold in aqua regia; but the second only explains also the reactions above referred to and taking place in the electrolytical decomposition of gold-chlorid solutions in the presence of gold electrodes.

If AuCl, were produced by the contact of chlorin with gold, this combination would resuit in the absence of hydrochloric acid or another chlorin compound when the chlorin is evolved at the anode under the influence of the electric current and a brownish-red liquid would flow along the anode. That this is not the case, but, on the contrary, that chlorin gas escapes at the anode in the absence of hydrochloric acid or other chlorin compound in the solution, contradicts the assumption that chlorin and gold alone in an aqueous solution can combine and form AuOl and yet the conditions during the evolution of chlorin at the anode are the most favorable that could possibly be imagined to the formation of this compound. On the contrary, in accordance with the equation Au-I-Oh-l-HOI: Au Ol H it follows necessarily that gaseous chlorin es capes Without acting on the gold anode when there is a deficiency of hydrochloric acid and that by adding a sufficient quantity of hydrochloric acid the. dissolution of the gold anode takes place. The above indivisible equation shows at the same time and quantitatively the factors which are important in the electrolytic dissolution of gold.

The volume of chlorin evolved per second at the anode is determined by the intensity of the current, and this volume is wholly absorbedthat is to say, utilized in the dissolution of gold when there is present at the anode a sufficiency of hydrochloric acid or another chlorin com pound, so that the amount of chlorin therein will be equal to at least one-third of that evolved at the anode, this following directly from the equation:

Au+Cl +IIOl:I-IAuOl If a less quantity of undecomposed hydrochloric acid is present, a corresponding proportion of chlorin must necessarily escape at the anode.

. Au-tCh-l-HCI:AuCLJI-i-O].

If undecomposed hydrochloric acid in sufficient quantity is present, so that the proportion of chlorin therein is greater than one-third of the chlorin evolved at the anode, it is possible with a current of higher density-that is to say, under a more copious evolution of chlorin at the anode per second-to bind the whole of the evolved chlorin-11 e., utilize the same for the dissolution of gold.

In a similar manner it follows quite naturally that under an increase of current density, hence under an increase in the volume of chlorin evolved within a unitof time upon a unit of anode-surface, a corresponding increase in the volume of hydrochloric acid will be necessaryin order that no chlorin may escape and that the uninterrupted dissolutionof the anode may take place.

:nAu+n(Ol )+n(HOl):MHAuOh).

It the electrolyte composed of AuOl H contains no free hydrochloric acid, then, according to Hittorf, (Poggendoryj s Annalcn der Ph siltand Chem la, Vol. 106, p. 522, et seq.,) the reactions proceed in such manner that the group AuC] will be separated at the anode and hydrogen at the cathode and gold only by the secondary effect of such separating hydrogen.

The group AuOl separated at the anode cannot subsist, and consequently breaks up according to the equation:

3. 3AuG1 :3AuOl +3Cl.

Hence chlorin is separated at the anode, in that a reaction in accordance with Au-tOl zAuCl cannot take place in an aqueous solution the greater part of the chlorin escapes, and. this I prevent, as stated, by the addition of another chlorin compound to the solution, because by this addition the conditions required for the dissolution of gold according to .are fulfilled.

and as a result gold will be dissolved. It is even for this reason that the aqueous solution of chlorin acts as a solvent of gold. The solution does therefore take place in both instances, not because chlorin is capable of dissolving gold without hydrochloric acid, but

because it is barely possibly to obtain a so-.

lution of chlorin free from hydrochloric acid. It is therefore obvious that an addition of hydrochloric acid will prevent the evolution of gaseous chlorin at the anode in the same manner and for the same reasons that a non-decomposed portion of the hydrochloric acid in the dissolution of gold in aqua regia serves to effect the combination of the gold with the chlorin.

In practically carrying out my invention I employ, by preference, as electrolyte a solution containing from about twenty-five to thirty grams of gold per liter and maintain the same during the electrolytic operation at a temperature of from about 60 to centigrade, which electrolyte may contain per liter from about twenty to fifty cubic centimeters,

. per square meter.

the electrolyte at the cathode is therefore according to the density of the current used, of fuming hydrochloric acid of a specific gravity of 1.19. In view of the fact that the proportion of hydrochloric acid may become variable during the electrolytic operation by reason of evaporation and consumption any deficiency in acid will manifest itself at once irrespective of any test as to acidity of the electrolyte by the escape of chlorin gas at the anode, and this is to be immediately remedied by the addition of hydrochloric acid until such escape ceases, provided there has been no material reduction in the temperature of the electrolyte. lVhen the temperature of the electrolyte and the'quantity of hydrochloric acid therein are in proper accord with the density of the electric current, the bulk of the foreign constituents of the anode will pass into the electrolyte under the influence of the electric current with the anode gold and with these the whole of the platinum and palladium. The major portion of the iridium and other metals of the platinum group remain undissolved, and with these the silver is separated as solid in the form of a chlorid. Similarly a portion of the lead is separated in the form of lead chlorid as soon as the electrolyte becomes saturated therewith, or nearly so, while the bismuth is separated in the form of an oxychlorid should the amount of hydrochloric acid present be insuflicient to keep it dissolved. With this insoluble mud, separated at the anode as described and which readily drops off the same in small quantities at a time, there is at all times mixed,irrespective of infinitesimal fractions of unchanged anode, a preponderating quantity (about ten per cent. of the weight of the anode) of gold in an extremely fine state of division. The presence of this gold is due to the fact that a portion of the anode gold is at once converted into a protochlorid (AuCl) instead of being first converted into a chlorid, and the protochlorid is immediately after being formed broken up into a chlorid and finely-divided gold.

While, as described, a portion of the anode constituents pass over into the solution and another portion subsides in the form of an insoluble deposit, pure gold is exclusively deposited at the cathode, even from solutions of constantly-increasing impurity and with currents of a density of one thousand amperes The gold extracted from constantly replaced principally by gold dissolved at the anode and in part by equivalent qnantities of other metals, and as the quantity of gold deposited at the cathode corresponds with the quantity of silver, lead, and bismuth converted into solid chlorids there is a gradual impoverishment of the electrolyte in gold, which is to be replaced by the addition of sufficient gold-chlorid solution.

The simultaneous accumulation of foreign constituents in the electrolyte does not affect the purity of the gold deposited, even if there is an excess of such impurities, so long as the proper amount of gold is present in the electrolyte. In fact, the presence of these foreign constituents is even advantageous to the deposition of pure gold at the cathode in that although the deposit is very voluminous from pure solutions, especially when currents of high density are used, yet the compactness of the deposit without being in the least affected in its purity increases with the increase in the impurities in the electrolyte.

From what has been said it will be seen that the electrolytic extraction of gold can be continuously carried on with the least possible quantities of hydrochloric acid, while in the purification of gold by chemical means fresh acid is required for each batch of material treated, and not only this, but each solution must be treated with fresh quantities of precipitant. In my process the small quantity of hydrochloric acid necessary to the formation of the electrolyte in consequence of the separation of pure gold at the cathode is always available anew as a solvent. Hence this very small quantity of acid will cause or insure in a greater sequence of operations not only the dissolution of the gold, but also the precipitation from the solution of the pure gold contained therein. There is a further advantage in the continuous extraction of gold from like solutions in that it is possible to recover from large quantities of anode materials the comparatively small percentage of platinum contained therein in that after the precipitation of the gold the platinum may be directly recovered in the form of ammonio-chlorid. Thus, for instance, if the anode metal contaius'on an average one-thousandth part of platinum two kilos of this metal would be obtained in a hundred days from a daily working up of twenty kilos of anode metal. In view of the fact, as has been shown by practice, that the electrolyte may contain twice as much platinum as gold without affecting the purity of the cathode deposit it is obvious that the electrolyte can be used until the separation therefrom of its platinum constituent can be most advantageously effected. This, however, is not the case with palladium, although quantities of this metal equal to five grams per liter of the electrolyte will not appreciably affect the purity of the gold deposited at the cathode. The presence of a greater quantity will, however,

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affect the purity of said deposit in that traces of palladium become apparent therein. In view of the fact that the proportion of palladium in the anode metal is usually but a fraction of its platinum contents the electrolyte can be used as such until it contains platinum enough to warrant its advantageous that the work be performed within the shortest possible time and within the smallest possible space. Accordingly currents of the highest possible density and comparatively thin anodes should be made use of. Thus, for instance, if an anode four millimeters thick is used it will be so far consumed in twenty-four hours with an initial current density of four hundred amperes per square meter that the remainder, of about one-tenth the original weight, is preferably used in making fresh anodes. I

The approximately uniform percentage of gold that goes over into the anode mud which collects on the bottom of the electrolytic apparatusis recovered therefrom either directly or after preliminary purification of the mud, according to its composition, and used as anode material. To this end the mud may be removed from the electrolytic apparatus either after each operation or after longer periods, as may be found necessary or advantageous.

The cathodes are made of thin rolled pure gold electrolytically obtained and may be of the same length as the anodes, but considerably narrower, because they rapidly increase in size in all directions under the action of the electric current, the deposit being a coherent one even with currents of a comparatively high initial density. This fact enables me, even with currents of high density, to place the electrodes in close proximity to each other, as within about three centimeters from each other, without danger of shortcircuiting or danger of the cathode deposit being rendered impure.

At the beginning of the electrolytic operation, when the electrolyte is yet free or comparatively free from foreign matter or impurities and the deposition of gold at the cathode is comparatively voluminous, the

. electrodes must be arranged farther apart or the cathodesmore frequently changed.

In the accompanying drawings I have illustrated an apparatus for practically carrying out my invention, showing the arrangement of the electrodes in their respective interconnected cells or vessels and combined with means for maintaining the level of the liquid in said cells or vessels, and in which Figure 1 is a vertical longitudinal sectional view of one of the vessels, and Fig. 2 a sectional plan view of a number of such connected in series.

Upon each vessel 1; for containing the electrolyte and electrodes is arranged a frame w, of wood, and upon the longer sides of said frame suitable conductive strips or bars a and b, of copperor brass, are secured, to which cross-bars, (numbered from 1 to 9, inclusive,) also of a conductive material, are alternately connected, and to said cross-bars the electrodes are electrically connected in such manner that the anode cross-bars 2 4 6 8 alternate with the cathode cross-bars 1, 3, 5, 7, and 9, connected with their respective conductive are indicated by the symbol A and the oathodes by the symbol C. The vessels '1) are connected in series through a conductive conncction 0 between the conductors b and a of two contiguous vessels, the conductors a and b of the first and last vessels of the series being respectively connected with the negative and positive poles of a suitable source of electricity, as a dynamo.

The cells or vesselso are made of any suitable material capable of resisting the action of the electrolyte, and if the latter is heated it is necessary, owing to loss by evaporation, to maintain it at a uniform level, to which end I arrange above thevessel '12 a reservoir f, containing an electrolyte such as is con-' tained in said vessel, and from said reservoir lead two pipes m and n, the reservoir being provided'with a normally-closed filling-aperture. -The pipe m has its outlet slightly below the surface of the electrolye in vessel 1), while the pipe nisa siphon-pipe that extends near to the bottom of said reservoir f and supplies liquid to vessel 1) the moment the level sinks below the outlet of pipe m and allows air to enter the same, while the flow of liquid from the reservoir is stopped the moment the level of the electrolyte has risen to seal the outletof said pipe. Theliquid used for compensating losses of the electrolyte by evaporation is preferably the water used in washing the gold deposits and the residues and mud taken from the cells, of which'water there is generallya sufficient quantity available; but should this not be the case the deficiency is made up with distilled water. In this manner I am enabled to return to the electrolyte the diluted portion of the gold solution taken therefrom without previously concentrating the same, as this is effectedby the evaporation of the hot electrolyte during the electrolytic operation. In the extraction of chemically pure gel from alloys of such it must be borne in mind that in the chlorination of the anode constituents other than gold an equivalent quantity of the latter metal is separated from the electrolyte at the cathode, which must be replaced by addition of gold chlorid. With such materials it will be necessary to supple ment the electrolytical process by chemical processes in proportion as the quantity of the components of the gold alloy otherthan gold is greater or less and as the electrochemical equivalent thereof relatively to gold is lower. If the anode material contains a comparatively great quantity of silver, the anode-surface will be coated therewith in the form of a non-conductive chlorid that does not readily drop off, thereby reducing the active area of the anode, giving rise to the evolution of gaseous chlorin, so that the anode will-be insufficiently attacked and the gold in solu* tion will be precipitated. In electrolyzing alloys of this kind theinterruption of the op: eration by the deposition of chlorid of silver strips (1 b by binding-posts d b. The anodes at the anode can readily be avoided by removal of said chlorid mechanically, as by means of a spatula, which operation is performed periodically, the frequency depending upon thepercentage of silver in thealloy and the density of the electric current. This also applies in case bismuth is present in the alloy, and if the latter also contains a considerable amount of lead the acid solution becomes rapidly saturated with lead chlorid, and as soon as this is the case both electrodes, as Well as the internal surfaces of the cell or vessel and the surface of the solution or electrolyte, become coated with lead-chlorid crystals. Although in this caseit is possible to obtain the precipitated gold free from lead by repeated boiling in distilled water and that the greater portion of the lead chlorid may be removed from the cold electrolyte by scooping it out, yet an electrolyte thus charged with impurities is not adapted for continuous operation. This can also be avoided by adding to the electrolyte a sufliciency of sulfuric acid to convert the lead chlorid into sulfate, and to this end I add to the electrolyte a quantity of concentrated sulfuric acid equal to the quantity of free hydrochloric acid present. The lead then forms on the anode an almost insoluble deposit of lead sulfate, which gradually drops off or which, as in the case of silver, is mechanically removed when lead is present at the anode in considerable quantities.

Of course itwill be understood that the sulfuric acid consumed in the formation of the' lead sulfate will have to be replaced from time to time and that whenever the anode material is .supposed to contain leacLit is advisable to add sulfuric acid to the electrolyte, especially as this acid does not exert a deleterious influence on the electrolytic actions.

Having thus described my invention, what 1 claim as new therein, and desire to secure by Letters Patent, is 1 1. The herein-described process,which consists in passing an electric current from an anode of impure gold or of an auriferous alloy through a'gold-chlorid solution, to a suitable cathode, and maintaining in said solution at all times during the passage of such current hydrochloric acid in su fficient quantity to prevent the liberation of gaseous ehlorin at the anode, for the purpose set forth.

2. The herein-described process,which eon-. sists in passing an electric current from an anode of impure gold or of an auriferous alloy through a heated gold-chlorid solution to a suitable cathode, and maintaining in said solution at all times during the passage of such current hydrochloric acid in sufficient quantity to prevent the liberation of gaseous chlorin at the anode, for the purpose set forth.

3. The herein-described process, which consists in passing a current of high density (five hundred am peres per square meter or higher) from an anode of impure gold or of an auriferous alloy through a gold-chlorid solution to a suitable cathode, and maintaining in said solution at all times during the passage of such current hydrochloric acid in sufficient quantity to prevent the liberation of gaseous chlorin at the anode, for the purpose set forth.

4. The herein-described process,which consists in passing a current of high density (five v hundred am peres per square meter or higher) from an anode of impure gold or of an auriferous alloy through a heated gold-chlorid solution to a suitable cathode, and maintaining in said solution at all times during the passage of such current hydrochloric acid in sufiicient quantity to prevent the liberation of gaseous chlorin at the anode, for the purpose set forth.

5. The herein-described process which consists in passing an electric current from an anode of impure gold or of an auriferous alloy to a suitable cathode through a gold-chlorid solution and maintainingin said solution sulfuric acid, together with hydrochloric acid in sufficient quantity to prevent the liberation of gaseous chlorin at the anode during the electrolytic action, for the purpose set forth.

6. The herein-described process,which consists in passing an electric current from an anode of impure gold or of an auriferous alloy through a heated gold-chlorid solution to a suitable cathode, and maintaining in said solution sulfuric acid, together with hydrochloric acid in suflicient quantity to prevent the liberation of gaseous chlorin at the anode during the electrolytic action, for the purpose set forth.

In testimony that I claim the foregoing as my invention I have signed my name in presence of two subscribing Witnesses.

EMIL VVOHLWILL.

Witnesses:

MARCO SALOMON, ADOLPH Button.

IOC 

